Natural gas is increasingly favored as an energy source over oil and coal because of its lower carbon emissions per unit of energy. Natural gas also emits significantly less nitrogen, sulfur, and heavy metals, among other pollutants, making it one of the cleanest burning hydrocarbon energy sources available on a commercial scale.
Most natural gas, including methane, is created by either thermogenic or biogenic geologic processes. Thermogenic generation involves the conversion of deep organic sediment material by extreme pressure and heat into coal, oil, and natural gas. Thermogenic generation of natural gas typically involves processes that occur on geologic timescales of thousands to millions of years. Thus like thermogenically formed coal and oil, thermogenically formed natural gas is extracted and utilized at much fast rates than it is created, disqualifying it as a renewable or sustainable energy source.
In contrast, biogenic generation of natural gas involves the activity of microorganisms metabolizing carbonaceous materials such as oil and coal into methane and other small molecule metabolic products on much shorter timescales. These microorganisms generally live amongst the carbonaceous material in environments with low concentrations of free molecular oxygen (i.e., anaerobic environments) and use metabolic pathways other than traditional aerobic respiration to live and grow. These pathways may include the breakup of the starting carbonaceous material into smaller hydrocarbon compounds, for example the fragmentation of smaller aromatic and/or aliphatic organic compounds (e.g., hydrocarbons) from a polymeric coal molecule. In an anaerobic formation environment, the pathways may further include the metabolism of the smaller organic compounds into the feedstocks of methanogenic activity. For example, the compounds may be converted by anaerobic fermentation into carbon dioxide (CO2), molecular hydrogen (H2), and acetate (CH3COO−) among other products. Finally, these metabolic products may be reduced by methanogenic microorganisms present in the formation to methane and other products by various methanogenic pathways.
While biogenic methane production in anaerobic formation environments has been known for some time, its contribution to commercial scale methane production has not been well understood. Unlike surface microorganisms that are readily studied in ambient air, most methanogens do not survive long in an aerobic environment. Great care must be taken to maintain these methanogens in an anaerobic environment, which makes it more difficult to extract and study them in a laboratory setting. Consequently, our understanding of the conditions that stimulate methanogen growth and activity is less developed than for many types of aerobic microorganisms.
Biogenic methane production may also be curtailed by conventional processes for recovering hydrocarbons from subsurface formation environments. For example, conventional processes for extracting natural gas from subterranean coal formations often remove a significant portion of the water from a coal seam to reduce pressure and allow previously adsorbed natural gas to flow up a well bore for recovery at the surface. As a result, the natural and ongoing biogenic production of methane can be interrupted, slowing or even halting biological methanogenesis while leaving a large mass of unutilized carbonaceous material remaining in the formation.
There is a need to, better understand how conditions in the anaerobic formation environment affect the growth and activity of methanogenic microorganisms. There is a further need to apply this understanding to the development of hydrocarbon recovery, processes that stimulate the generation and recovery of biogenic natural gas on a commercially significant scale. These and other topics are addressed in the present application.